1. Field of the Invention
This invention relates to gyroscopes in general although, for convenience, will in the main be discussed in relation to flexure-suspended, free-rotor gyroscopes since the invention is particularly applicable thereto.
2. Description of the Prior Art
A gyroscope having a sensitive element attached to a rotor of a hysteresis or synchronous motor suffers from a problem in that the hysteresis motor gives rise to vibration of the sensitive element with the result that a bias drift is introduced into the gyroscope, thereby degrading its performance. In the case of a four-pole hysteresis motor, stator coils thereof are arranged around the circumference of the rotor and are energised by sine and cosine waveforms which generate a pole or flux vector (V.sub.p) around the rotor circumference which rotates at half the frequency of the sine and cosine waveforms. Energisation of the stator coils causes a rotating magnetomotive force to accelerate the motor up to full speed. During normal running the angular relationship (.phi.) between an arbitrary vector AB fixed in the motor body and the axis of the rotating magnetising field MN is nominally constant. De-energising the stator coils momentarily whilst the rotor is spinning causes the rotating magnetising field vector MN to re-form at a different angle .phi. to AB.
Inevitable manufacturing and assembly tolerancies associated with hystereses motors mean that the air gap between the rotor and stator is not entirely uniform, hence there is likely to be a point on the the stator ring that is closer to the motor spin axis than any other point on the ring. If a diameter CD is drawn through this point, it will always nominally be fixed with respect to the stator windings.
The rotor, being made of magnetic material, is subject to the forces of magnetic attraction and nowhere is this attractive force so great as when the magnetising field parallel to MN bridges this point of smallest air gap, i.e. when MN is parallel to CD. This attractive force is along CD and occurs twice per rotor revolution and always in the same direction with respect to the stator. An observer fixed to the stator at the point of smallest air gap measuring both the attractive force and the magnetising field would see a sinusoidal magnetising field and a fluctuating or pulsating unidirectional attractive force. This attractive force sets up vibrations which cause the gyroscope to drift. The points on the rotor at which the maximum force is experienced can be indicated by the intersection of MN with the rotor circumference. These points, together with the rotor spin axis, define a plane along which the maximum amplitude pulsations occur and changing the angular relationship .phi. shifts the points, and hence the plane of vibration, which is sufficient to change the value and direction of the bias drift of the gyroscope. Thus the value of bias drift varies from switch-on to switch-on of the gyroscope. It is this lack of bias drift repeatability which is the problem because if the drift were constant, then the necessary correction can be made during calibration.
This change in bias drift, which can be of the order of several degrees per hour, introduced by the hysteresis motor is unacceptable in any high quality gyroscope but is particularly problematical in a flexure-suspended, free-rotor gyroscope such as disclosed, for example, in British Pat. Nos. 1,304,571 and 1,364,757.
There are some known methods of alleviating the identified problem one of which is to employ a servo loop in the gyroscope drive which ensures that the rotational speed of the rotor is slightly different from that which would pertain if the rotor were in synchronism with the rotating vector MN. This method is effective in overcoming the problem but consumes a relatively large amount of power which can be critical in some applications. Another known method is to change the phase of the stator energising signals, by interrupting the signals, so as to change sequentially the direction of the axes of the permanent magnets formed in the rotor and hence the direction of the bias drift. In this way, the bias drift is averaged out. Again this method is effective in meeting the problem concerned but it has now been realised that it is the cause of another problem. Every time the stator energising signals are phase slipped, torque transients are introduced into the rotor which in turn causes a drift in the output signal of the gyroscope.
It is the object of the present invention to overcome the problem of changes in bias drift introduced by the hysteresis motor without degrading the performance of the gyroscope in other respects and without incurring excessive power consumption.